I would like to use a Raspberry Pi to interact with devices via Bluetooth, specifically Bluetooth Low Energy.
My understanding is that this is a protocol which was introduced with BT 4 but I do not understand whether special dongles are required. None of the ones I looked at mention Bluetooth Low Energy.
Are all BT devices (specifically USB dongles) automatically compatible with BLE if they are BT 4+?
There are substantial technical differences between BT4.0 "classic" and BLE. (See Bluetooth_Low_Energy: Technical_details on Wikipedia) which makes them distinctly different modes of operation. A single radio can cater for both, but the software would have to support the different styles of connection.
From the compatibility section on that Wikipedia page:
Bluetooth Low Energy is not backward-compatible with the previous (often called "classic") Bluetooth Basic Rate/Enhanced Data Rate (BR/EDR) protocol. The Bluetooth 4.0 specification permits devices to implement either or both of the LE and BR/EDR systems.
So no, BT 4.0 does not mandate or imply BLE support. Devices can support both, but stating support for one does not mean that support for the other is guaranteed.
Similarly for Bluetooth 5 from Summitdata: What's new in Bluetooth 5
All the major enhancements in Bluetooth 5 are for the BLE radio and there is nothing new associated with the Bluetooth Classic radio.
The "Classic" Bluetooth radio and the BLE radio are still separate in Bluetooth 5, with the implication that "Bluetooth 5" means "classic bluetooth" and if BLE is stated along with Bluetooth 5 then it will be the enhanced "Bluetooth 5 BLE".
